#13341: Lift constraint products -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I want to get a discussion going, could we lift constraint products automatically. Although not partially applicable, we can consider `(,)` as having kind {{{#!hs (,) :: Constraint -> Constraint -> Constraint }}} I propose also giving it kinds {{{#!hs (,) :: (k -> Constraint) -> (k -> Constraint) -> (k -> Constraint) (,) :: (k -> k' -> Constraint) -> (k -> k' -> Constraint) -> (k -> k' -> Constraint) -- etc. }}} == Translation {{{#!hs (Eq, Num, Show) (Monad, Foldable) (Category, Profunctor) }}} gets turned into something like {{{#!hs class (c a, d a) => (c :&: d) a instance (c a, d a) => (c :&: d) a infixl 7 :&: Eq:&:Num:&:Show Monad:&:Foldable Category:&:Profunctor }}} == Uses Very often I need to be able to combine constraints. A small modification of [https://hackage.haskell.org/package/free- functors-0.7.2/docs/Data-Constraint-Class1.html SuperClasses] {{{#!hs type c :~ d = forall xx. c x :- d xx class HasSuperClasses (c :: k -> Constraint) where type SuperClasses c :: k -> Constraint type SuperClasses c = c superClasses :: c :~ SuperClasses c containsSelf :: SuperClasses c :~ c instance HasSuperClasses Functor where superClasses :: Functor :~ Functor superClasses = Sub Dict containsSelf :: Functor :~ Functor containsSelf = Sub Dict instance HasSuperClasses Foldable where superClasses :: Foldable :~ Foldable superClasses = Sub Dict containsSelf :: Foldable :~ Foldable containsSelf = Sub Dict }}} I want to be able to write {{{#!hs instance HasSuperClasses Traversable where type Traversable = (Traversable, Functor, Foldable) superClasses :: Traversable :~ (Traversable, Functor, Foldable) superClasses = Sub Dict containsSelf :: (Traversable, Functor, Foldable) :~ Traversable containsSelf = Sub Dict }}} If this doesn't pose any difficulties I'll open a GHC proposal. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13341 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler